Linux kernel 3.6 has been released. There are new features in Btrfs: subvolume quotas, quota groups and snapshot diffs (aka "send/receive"). It also includes support for suspending to disk and memory at the same time, a TCP "Fast Open" mode, a "TCP small queues" feature to fight bufferbloat; support for safe swapping over NFS/NBD, support for the PCIe D3cold power state; and VFIO, which allows safe access from guest drivers to bare-metal host devices. Many small features and new drivers and fixes are also available. Here's the full list of changes

One of the most interesting changes, from a "philosophical" point of view, is VFIO, because it allows to create safe and fast user-space drivers. So people who likes microkernels don't even need to write a OS from scratch to do experiments.